In many industrial applications, it is desirable to produce articles having an inexpensive and lightweight material constituting the core; such materials will typically be non-allotropic metals including aluminum. As earlier indicated, non-allotropic metals shall mean herein non-transformation hardenable metals having a hardness less than R.sub.c 25. The surface of such articles must also possess and have physical properties not provided by the core material itself. Such enhanced physical properties may include high hardness, high strength, elevated temperature wear resistance, and corrosion resistance.
Some form of new surface treating technology must be generated to achieve such properties in a precisely selected surface zone without affecting the non-allotropic metal core; it cannot be achieved reasonably economically by applying presently known surface treating technology. Known treating technology comprises: (a) saturating the surface zone, as by carburizing or nitriding, (b) transforming the surface zone solidification phase to one which is harder, (c) attaching a coating, or (d) alloying or heat treating the entire article. Nitriding and carburizing are employed with success for iron-base substrates, but are not successful with non-ferrous materials. Transformation hardening is quite successful with iron-base substrates, but it is not successful with aluminum and many other non-allotropic materials. Attached coatings are expensive and may lack permanence. Treating the entire article is wasteful of energy, is low in productivity and fails to achieve differential characteristics in the core and surface zone. For example, with an aluminum article and the like, the prior art has principally employed precipitation hardening throughout the entire article. This method is unsatifactory for a variety of reasons including high cost, distortion, and low productivity. Little or no work has been performed with respect to surface region treatments of aluminum and no work has been performed with respect to utilizing a highly concentrated energy beam as one of the factors in such surface treatment technology.
High intensity energy heat sources have been employed for purposes of welding, cutting and drilling, and in certain limited modes for the purpose of surface hardening of ferrous-based materials. The high energy beam can be employed to melt a very shallow surface region of an iron based article with the result that the melted material can be transformed to a harder phase upon removal of the energy beam, allowing the article to perform as a self-quenching medium. However, the technique of using a high energy beam for surface hardening ferrous-based material is totally different than its use when applied to non-ferrous and particularly non-allotropic materials.
Little or no thought has been given to the concept of controlling the introduction of alloying ingredients to controlled depths and proportions within a non-allotropic metal base, such as aluminum, by the use of a high energy beam. The lack of investigation may be attributed to the prevailing thought that the usefulness of such a beam, when applied to aluminum, would be limited because (a) melting typically does not lead to a hardened transformed phase within such material, (b) past experience with furnace heat treatment indicated limited hardness levels to which many non-allotropic metals could be hardened, (c) the lack of commercial need to investigate how to deep harden localized zones with little distortion, and (d) the availability of alternate hardening techniques for commercial needs which usually were shallow non-severe wear surfaces, one technique being plasma spraying which did not distort the substrate and was very flexible in use. Thus, utility of a highly concentrated energy beam had not been envisioned in applications involving aluminum and the like.
Particularly with respect to aluminum, one or more of the following disadvantages may occur with present technology: (a) the article may be highly distorted as a result of the hardening treatment, (b) the surface contour of the part to be treated may be irregular and therefore is not susceptible to uniform treatment or the article may have different sections and the different sections respond differently to the hardening treatment causing non-uniformity, (c) the cost of hardening an aluminum article may be relatively high due to the requirement for expensive equipment or manpower, (d) the method of heat treatment is unable to achieve a shallow uniform case depth with accuracy, (e) the method of treatment is unable to achieve selective precision patterns of case hardening over a given surface, (f) the prior art method is unable to economically harden a small area of a large sized article, (g) the prior art method is unable to harden small areas which are difficult to reach within a complex part, (h) the prior art method is unable to be used without potential damage of adjacent parts, (i) quenching becomes difficult at best with certain of the prior art methods, and (j) the prior art methods do not lend themselves to extremely high volume and fast rates of production. Accordingly, there is a need for a method of surface treating aluminum articles, and the like, which overcomes the above problems and in addition improves the surface treating technology for non-allotropic materials to facilitate achieving all desirable physical properties with adequate control.